The use of communication systems having wireless mobile communication units has become widespread. Wireless communication systems provide several important advantages over conventional wired systems. For example, wireless communication users can communicate in locations where wired service is not available or feasible, such as remote or rugged locations. Additionally, wireless communication users have much more mobility because they do not have to connect to a fixed wired network. These and other favorable characteristics make wireless communications ideally suited for personal, business, military, search and rescue, law enforcement, and water vehicle applications.
Common approaches to providing wireless communication systems are two-way radio systems and cellular systems. Cellular systems require fixed based stations such as cell towers that are coupled to a wired network. These types of systems are not practical when the entire network needs to remain mobile. Typically, two-way radio systems contain their own self-contained infrastructure, such that all of the mobile communication units (e.g., radios, data terminals, voice/data terminals) within a network need to be programmed to similar frequencies or channels.
Despite the benefits of two-way radios, they have several drawbacks. First, two-way radios typically support only half-duplex operation, meaning that only one user can talk at a time. A user must push a “talk” button to talk to another user and if both users push their “talk” buttons at the same time, then the users cannot hear each other. The reason for this is that all participants transmit data on the same channel. Two-way radios also require that users agree upon and select a particular channel to use for a conversation. A user can only talk to others tuned to the same parameters (e.g., frequency, modulation type, cryptographic settings, frequency-hopping settings). Typically, this means that a user can only talk to others that have similar types of radio equipment. If the channel being used is lost, or cannot be used because of excessive interference, then the users must transfer to a different channel, which in practice can be difficult to do because even if an alternate channel has been previously agreed upon it is often difficult to create a reliable method for all parties to determine that the original channel is no longer available.
Two-way radio systems are often line-of-sight-type radio systems. Military line-of-sight-type radios are typically VHF (Very High Frequency) or UHF (Ultra High Frequency) radios that broadcast in frequencies from about 30 MHZ to 300 MHZ, and have effective ranges of about 5-25 miles, based on RF power, the antenna/mast height used with the radio, and other factors. These VHF/UHF radio types have limited range capabilities, but are used because these radio bands can be highly reliable. Other radio types can at times provide beyond-line-of-sight service, for example, HF (High Frequency) radios. HF radios broadcast in frequency ranges from about 1 MHZ to about 20 MHZ, and can at times achieve much longer ranges than VHF/UHF radios. However, the HF radio broadcasts are much less reliable than the VHF/UHF radio broadcasts.
Current line-of sight voice communication systems are broadcast on dedicated networks. The radios are set in frequencies that operate only in the line-of-sight, as these provide high reliability within the line-of-sight constraints. These radio types are typically push-to-talk, half-duplex systems that allow a person to talk only to other radios that are operating within the same physical network. While highly reliable within these constraints, it is highly desirable to allow direct half-duplex voice communications to occur with different groups or organizations across different logical networks.